EXERCISE 6-1 Ball milling circuit using Austin's model - Limestone

Simulate a ball milling circuit that must handle 100 tonnes/hr of ore. The largest particle size in the feed is 10 mm. The feed has a Rosin-Rammler size distribution with D63.2 = 2.5 mm and lambda = 1.2. The specifications of the equipment are

Ball mill

Mean residence time 7 minutes

Overflow discharge with no overfilling and no post classification. 70% solids in the mill.

Cyclones

38 cm diameter "standard" geometry with 10 cyclones in the cluster. Sufficient water should be added to the sump to make the cyclone feed 45% solids by weight.

The circuit is standard with preclassification of feed. A suitable flowsheet is illustrated in Figure 1.

Name the plant feed stream, the ball mill feed stream, the ball mill product stream and the cyclone overflow stream for future reference. Simulate the operation of this circuit when it processes limestone. Parameters that define the selection function for limestone have been determined and are preloaded for selection using the model GMIL in MODSIM. Limestone has a specific gravity of 2.7 and Bond Work index of 11.1 kWhr/tonne.

Use the preset parameters for the selection function and the breakage function. These parameters can be changed by the user to suit any particular material but for the purposes of this exercise the default values should be used.

Edit the parameters for the hydrocyclone. Choose the Plitt model CYCL. Set the cyclone diameter to 38 cm and specify 10 cyclones in the cluster. Leave all other parameters at their default settings.

Run the simulation and make a note of the following information.

1. Recirculating load = 100xball mill feed/plant feed

2. Cyclone underflow % solids

3. D80 in the mill feed and product. Calculate the power required using Rowland's factor for Bond work index in open circuit.

4. D50 in the cyclone

5. Pressure drop across the cyclone

6. Right click on the mill icon to generate a plot of the selection function that was generated by the mill model. Save a copy of this graph for future reference.

7. Plot the size distributions in the plant feed, the ball mill feed and the cyclone overflow.

View the size distribution graphs for the plant feed, the ball mill feed, the ball mill product and the cyclone overflow make a pattern that gives an immediate diagnosis of the health of the milling circuit. The graphs for this case are shown below. Note the spread between the plant feed and plant product graphs and the much narrower spread between the ball mill feed and the ball mill product. This shows the additional size reduction that is achieved by the circulating load over and above that generated by the ball mill itself. In a well designed and operated ball mill circuit the inner mill envelop should be entirely within the outer circuit envelope. In this example the plant and mill feed lines cross at about 4 mm indicating that >4 mm material is accumulating in the circuit which is symptomatic of a media ball size that is too small for the material that is processed.

The graphs for this case are shown below

Save the job.